1) There were also many corps pre-DCI that were not able to manage to budgets, due to the very shoe-string nature of the activity. Corps failed all the time through the 50's and 60's pre-DCI. One difference from later on is that if a corps failed in one place, another sprung up nearby. Once that slowed own, the corps numbers started to decline.
2) There were also many staff members who WERE scholastic music teachers pre-DCI. Many were not, but many were.
3) True
4) Not to mention scholastic-sponsored fund-raising as opposed to a mysterious "outside" activity like drum corps. School budgets are often paying stipends to some staff, just like coaches, but at least around me most of the design and props and stuff is not funded by the district, but by the band parents efforts. Bands with large staffs of techs, etc, also use parent-raised money to pay those people.
5) Yup. Agree.